According documentation, proxy in most contexts must 
have the same behaviour as object itself.

PROBLEM:

bool(proxy_object) != bool(object_itself)

EXAMPLE:

>>> class X(list): pass # collection class
... 
>>> x = X() # empty collection
>>> import weakref
>>> proxy = weakref.proxy(x)
>>> bool(x)
False
>>> bool(proxy)
True

PYTHON VERSION:

2.4 (#60, Nov 30 2004, 11:49:19) [MSC v.1310 32 bit 
(Intel)]

also tested on:

2.3.4 (#53, May 25 2004, 21:17:02) [MSC v.1200 32 bit 
(Intel)]

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The bug is in weakrefobject:proxy_nonzero(), which calls the underlying object's nb_nonzero slot instead of going through the general PyObject_IsTrue() mecanism.  Built-in types like list and dict don't have a nb_nonzero slot.

As a related bug, (Callable)ProxyType should implement tp_richcompare in addition to tp_compare.  In fact, we should review the code to check if ProxyType knows about the most recent type slots...

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Fixed the __nonzero__ problem for Py2.5 and will backport to
Py2.4.  See Objects/weakrefobject.c 1.21.

Leaviing this report open until the rest of the module can
be checked and fixed.

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The following type slots appear to be missing in the proxy types:

* nb_(inplace_){floor,true}_divide
* sq_repeat, sq_concat, sq_item, sq_ass_item, sq_inplace_concat, sq_inplace_repeat (which are needed for operator.repeat(), operator.concat(), etc. to work)
* tp_as_buffer
* tp_richcompare (tp_compare alone is not enough)
* tp_descr_get, tp_descr_set (for proxies to descriptors...)
* nb_coerce (though it seems that only an explicit call to coerce() can show that this is missing; implicit coercion is done correctly in other operators)
* nb_oct, nb_hex

As far as I can tell, there is no existing operator that is broken in the same way that nb_nonzero was.

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Also see bug #1075356 (which is very minor, but might as
well get fixed at the same time).

Raymond: Is this still a problem, or should we close it because the
reported issue is resolved and 2.4 is no longer maintained?

I'll close this in a couple of weeks unless anyone objects.

All the missing type slots I reported can cause incorrect behavior very similar to the one reported originally.  For example (in Python 2.7):

    class I(int): pass
    i = I(123)
    hex(i)                   => '0x7b'
    hex(weakref.proxy(i))    => TypeError

I think that the present bug report is fine to report these as bugs, as I don't see the point of opening another bug report with the same title.  I will just re-mark this issue as "open - not resolved".

Works in 2.7 and 3.2.

Not for me (the last example I posted, on 2.7 head).  But I will not fight for this.